Feature Request: Add faces for ESNext tokens

[elzii]

It would great if JS2 mode could define some faces for some ES2015/7/Next/whatever tokens. For example, the js2-ARROW and js2-TRIPLEDOT, ( => and { ...prop } respectively)

This would give people the ability to supplement syntax highlighting themes easily, and handle more special cases. Coming from other editors for example, the import keywords are often colored different than a const keyword, but both default to font-lock-keyword-face.

New to lisp, so if there's any easier way to do it I'd love to know!